Living organisms depend on each other for their food requirement and form a food chain.
Through such food chain energy and nutrients move to upper trophic levels sequentially. If any non-biodegradable substance enters in biotic factor of an ecosystem, its concentration progressively increases at higher trophic level.
The uncontrolled use of pesticides and other chemicals increases the amount of these chemicals in the soil or water bodies. From soil or water they enter in the body of plants and then in the bodies of herbivores and carnivores. As these chemicals are not degradable, they get accumulated progressively at each trophic level. This phenomenon is known as Biological magnification.
As human beings occupy the top level in any food chain, the maximum concentration of pesticides gets accumulated in human body. Therefore, though for increase of crop production we use such chemicals, we need to control their use.